Film molding method

Abstract: A film molding method and apparatus capable of shortening a mold cycle time and obtaining a molded product of an excellent quality. A method of hot-press molding a polyparaphenyleneterephthalamide (PPTA) film into a predetermined shape includes a first step of evaporating water from a PPTA film containing water at least 50 % or more as a swelling agent, and obtaining an amorphous film having a density less than a predetermined density and a modulus in tension of about 70 %; and a second process of hot-press molding the film for a predetermined time period by metal molds heated up to at least 330 °C to crystallize the film to have a predetermined density specific to a PPTA film and produce an intrinsic physical value, and obtaining a molded product of the film. A film molding apparatus for performing the film molding method includes holes or slits (1h) opening at the surface of each of the metal molds (1a,1b), and an air suction unit and an air exhaust unit provided for each of the metal molds.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for hot-press molding a para-orientation aromatic polyamide film, particularly a typical polyparaphenyleneterephthalamide (hereinafter called "PPTA") film, into a predetermined shape, capable of efficiently molding a film with a good quality. Related Background Art A para-orientation aromatic polyamide film, particularly a PPTA film, has an excellent acoustic property. The present inventors have proposed an acoustic vibration member made of a molded PPTA film and methods of molding a PPTA film (Japanese Patent Laid-open Publication No.3-38200). According to the first method of these molding methods, a PPTA film in a gel state containing water at least 50 % or more, or preferable 80 % or more, as a swelling agent and having a 100 % modulus in tension is hot-press molded by a hot pressing machine, and is thermally fixed while clamping the film by metal molds. According to the second molding method, a PPTA film thermally treated in an unrolled state to have a modulus in tension of about 40 % or more is hot-press molded by a hot pressing machine. Using the first and second molding methods, mass production of acoustic vibration members has been performed conventionally. The outline of the first method will be described with reference to Figs.9 to 12. A gel film Fg containing water at least 50 % or more, preferable 80 % or more, as a swelling agent, is clamped by metal molds 1u and 1b heated to about 150 °C, and thereafter the molds are heated up to 350 °C or more. Thereafter, the upper mold 1u is cooled down to 140 to 150 °C and the lower mold 1u is cooled down near 150 to 160 °C. Lastly, the metal molds 1u and 1b are released to dismount the molded film. More particularly, as shown in Fig.9, the gel film Fg is introduced to the intermediate of the upper and lower metal molds 1u and 1b. As the metal molds lower, the heater blocks 2 heated to about 380 to 400 °C move to attach the back surfaces of the upper and lower metal molds 1u and 1b so that the metal molds are heated and the gel film Fg is clamped by the metal molds. The temperature of the upper and lower metal molds 1u and 1b at the time when the clamping starts is 140 to 150 °C. After the clamping, the metal molds 1u and 1b immediately raise their temperatures because the heater blocks 2 heated to 380 to 400 °C contact the metal molds. As the temperatures of the metal molds 1u and 1b rise, water contained in the film Fg vaporizes in an accelerated manner and is expelled out of the metal molds 1u and 1b. When the metal mold temperature rises and reaches a predetermined temperature (350 °C or more), the film eventually undergoes a thermal process so that the film is crystallized to have a predetermined density specific to a PPTA film and produce an intrinsic physical value. However, although a PPTA film has a high elastic modulus, it takes a lower elastic modulus at such a high temperature of 350 °C or more. Releasing the metal molds at such a high temperature may deform the molded PPTA film by friction between the metal molds 1u and 1b and the film when the latter is dismounted from the metal molds. In order to avoid the deformation by friction, it is therefore preferable to lower the film temperature by recovering the intrinsic modulus of the PPTA film. For this reason, while the metal molds 1u and 1b maintain clamping the film, the heater blocks 2 detach from the bottom surfaces of the upper and lower metal molds 1u and 1b, and as shown in Fig.11, cooler blocks 2c slide and move to tightly contact the bottom surfaces of the upper and lower metal molds 1u and 1b. Cooling air is jetted out of holes formed in the cooler blocks 2c to cool the upper and lower metal molds 1u and 1b to a predetermined temperature (140 to 150 °C). Thereafter, as shown in Fig.12, the upper and lower metal molds 1u and 1b are released to dismount the molded film, and the cooler blocks return to the predetermined positions. The outline of the second molding method will be described with reference to Figs.13 to 16. A hard film Fh thermally treated to have a modulus in tension of 40% or more in an unrolled state is introduced to the intermediate of upper and lower metal molds 1u and 1b. Heater blocks 2 are made in tight contact with the bottom surfaces of the metal molds 1u and 1b and the metal molds 1u and 1b heated to about 350 °C clamp the film Fh as shown in Fig.14. This state is maintained for a predetermined time period (10 to 30 seconds). Thereafter, the temperature of the film is lowered in order to avoid a deformation of the film by friction when the film is dismounted as described previously. Specifically, while the metal molds 1u and 1b clamp the film, the heater blocks 2 detach from the bottom surfaces of the upper and lower metal blocks 1u and 1b. Thereafter, as shown in Fig.15, cooler blocks 2c slide to tightly contact the bottom surfaces of the upper and lower metal molds 1u and 1b. Cooling air is jetted out of holes formed in the cooler blocks 2c to cool the upper and lower metal molds 1u and 1b. As shown in Fig.15, when the upper and lower metal molds 1u and 1b have been cooled down near to 80 °C, the upper and lower metal molds 1u and 1b are released to dismount the molded film therefrom, and the heater blocks 2 and cooler blocks 2c are returned to the predetermined positions. The first and second molding methods have the same process that the molded film heated up to 350 °C is cooled while it is clamped by the upper and lower metal molds 1u and 1b. As stated before, a PPTA film heated to 350 °C or more has a lower elastic modulus. Therefore, if the metal molds are released immediately after the molding, the film has a strength insufficient for maintaining the molded shape, and is likely to be deformed by friction between the film and the molds when the molds are released or the film is dismounted from the molds. It is therefore necessary to cool the film until it has a strength resistant to such friction between the film and the molds when the molds are released or the film is dismounted from the molds. In the first and second molding methods, the necessary condition is to heat the metal molds to 350 °C and cool them down to 160 °C or near to 80 °C. This condition elongates one mold cycle time. Presently, the mold cycle time is about 5 minutes at the shortest, and under poor maintenance of the molding machine, it takes about 7 to 8 minutes. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problem and provide a film molding method capable of shortening a mold cycle time and obtaining a molded film having an excellent quality. In order to solve the above problem, the present invention provided the film molding method which basically includes a first process and a second process. The first process is a process of removing water contained in a gel PPTA film as a swelling agent, and the second process is a thermal treatment process for thermally fixing the gel PPTA film and producing an intrinsic performance of the film. By dividing the method into the first and second process, a cooling time in one mold cycle becomes zero. According to the first film molding method of this invention, in a film molding method of hot-press molding a film made of substantially para-oriented aromatic polyamide having a modulus in tension of 600 Kg/mm 2 or more, particularly a typical PPTA film, into a predetermined shape, the first process evaporates water from a PPTA film containing water at least 50 % or more, or preferably 80 % or more, as a swelling agent, and obtaining an amorphous film having a density less than a predetermined density and a modulus in tension of about 70 %., and the second process hot-press molds the film for a predetermined time period by metal molds heated up to at least 330 °C, or preferably 350 °C or more to crystallize said film to have a predetermined density specific to a PPTA film and produce an intrinsic physical value, and obtaining a molded product of the PPTA film. In this case, the film obtained by the first process is preheated at 200 °C or more to 300 °C or less after the first process. According to the second film molding method of this invention, in a film molding method of hot-press molding a PPTA film, the first process hot-press molds a PPTA film containing water at least 50 % or more, or preferably 80 % or more, as a swelling agent, by metal molds heated to 220 °C or less, or preferably 190 °C or less for a predetermined time period, and the second process hot-press molds the film for a predetermined time period by metal molds heated up to at least 330 °C, or preferably 350 °C or more to produce a PPTA molded product in the manner like the second process of the first film molding method. By dividing the method into the two processes, a cooling time for one mold cycle becomes zero. However, without the cooling process, the metal molds are released immediately after the heat process, i.e., at a temperature of 350 °C or more, the film is likely to be deformed. In view of this, each of the male and female metal molds are provided with an air suction unit and an air exhaust unit so as to make a film tightly attached to one of the metal molds when they are released. The film molding method of this invention can omit a cooling process, resulting in a greatly reduced one mold cycle time. Furthermore, since the metal molds are released while a molded film is tightly attached to the one of the metal molds, there is no fear of deformation of the film to be caused by friction when the metal molds are released, even if the film takes a high temperature, thereby obtaining a molded product of a high quality. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a front view in section of a molding machine used for a film molding apparatus according to an embodiment of the present invention. Fig.2 is a side view in section of the molding machine explaining a first molding method with a gel film being disposed at the intermediate of metal molds. Fig.3 is a side view in section of the molding machine explaining the first molding method with the gel film being clamped by the metal molds. Fig.4 is a side view in section of the molding machine explaining the first molding method with the upper metal mold being released. Fig.5 is a side view in section of the molding machine explaining the first molding method with a primary molded film being dismounted from the lower metal mold. Fig.6 shows the arrangement of the film molding apparatus using the first molding method. Fig.7 is a side view in section of the molding machine explaining a second molding method with a hard film being disposed at the intermediate of metal molds. Fig.8 is a side view in section of the molding machine explaining a third molding method with a hard film being disposed at the intermediate of metal molds. Fig.9 is a side view in section of a conventional film molding machine. Fig.10 is a side view in section of a conventional molding machine with a gel film being clamped by metal molds. Fig.11 is a side view in section of the conventional molding machine wherein the gel film has been molded, the metal molds have been released, and the molded film is dismounted from the metal molds. Fig.12 is a side view in section of a conventional molding machine where a gel film forming has been completed and the metal molds have been released. Fig.13 is a side view in section of a conventional molding machine with a hard film being disposed at an intermediate of metal molds. Fig.14 is a side view in section of the conventional molding machine with the hard film being clamped by the metal molds. Fig.15 is a side view in section of the conventional molding machine with the hard film being molded. Fig.16 is a side view in section of the conventional molding machine wherein the hard film has been molded, the metal molds have been released, and the molded film is dismounted from the metal molds. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a film molding method according to the present invention as well as a molding machine using the embodiment method will be described with reference to Figs.1 to 8. As shown in Figs.1 and 2, metal molds 1u and 1b are mounted on heater blocks 2. In this embodiment, the upper metal mold 1u is mounted at the end of a pressure cylinder S, whereas the lower metal mold 1b is mounted on the molding machine on its base B side. The upper and lower metal molds 1u and 1b are formed with fine holes or slits 1h. As shown in Fig.2, these holes 1h are guided to the outside of the metal molds, and connected via switches 3 to air suction units 4 and air exhaust units 5. The arrangement of the film molding apparatus used with the molding machine is shown in Fig.6. This film molding apparatus has a material supply unit Ms, molding machines explained in Figs.1 and 2, a trimming press PT, and a robot R. Three molding machines are used, two machines being used as primary molding machines P1, and one machine being used as a thermal treatment molding machine PH. The first molding method molds a gel film containing water at least 50 % or more, preferably 80 % or more, as a swelling agent. The gel film Fg used contains water 80 % or more as a swelling agent, and has a thickness of about 140 µ (finished thickness of 25 µ) and a modulus in tension of about 100 %. The film Fg is formed in a roll having at a predetermined width, and immersed in a water vessel of the material supply unit Ms so as not to evaporate water as the swelling agent. A necessary amount of the film Fg is fed out of the material supply unit Ms and the film Fg is cut to have a predetermined length. The cut film Fg is picked up by a robot arm Ra of the robot R and transported to the primary molding machine P1. As shown in Fig.2, the transported film Fg is set at the intermediate of the upper and lower metal molds 1u and 1b, and as shown in Fig.3 it is hot-press molded by the upper and lower metal molds 1u and 1b both heated to 200 °C. The pressure cylinder S is lowered, and immediately before the film Fg contacts the upper metal mold 1u, the switch valves 3 connected to the upper and lower metal molds 1u and 1b are switched to the air suction units 4 and at the same time the air suction units 4 are operated. In this condition, at the same time of the clamping start, water contents attached to the surface of the film Fg are sucked and exhausted out of the metal molds. The metal molds are further lowered, and when they completely clamp the film Fg, the latter is molded to have a shape matching the inner wall of the metal molds. At this complete clamping state, water contained in the film Fg is evaporated by the heat of the metal molds, and exhausted out of the holes 1h formed in the upper and lower metal molds 1u and 1b via the air suction units 4 into the outer air in about one to two minutes. After water contents contained in the film Fg are completely driven away from the film Fg, the upper and lower metal molds 1u and 1b are released. In this state, although the film Fg has been molded into a predetermined shape, the gel film Fg with its water contents having been removed is still in an amorphous phase and remains as a molded hard film Ff. This molded hard film Ff still does not provide the physical value specific to a PPTA, and therefore has insufficient mechanical strength. In the following, this hard film Ff is called a primary molded film h1. The hard film Ff having insufficient mechanical strength is likely to be deformed. If the metal molds are released under this condition, deformation by friction generated when releasing the metal molds becomes greater. In order to avoid such deformation, as shown in Fig.4, the air suction unit 4 for the upper metal mold 1u is stopped, and the switch valve 3 is switched to the air exhaust unit 5. Under this condition, the air exhaust unit 5 is operated and the cylinder 3 is raised to release the upper and lower metal molds 1u and 1b. Specifically, air exhausted from the upper metal mold 1u peels off the primary molded film h1 tightly attached to the upper metal mold 1u, and in addition, the primary molded film h1 is being tightly attached to the lower metal mold 1b by the continued operation of the air suction unit 4 for the lower metal mold 1b. Under this condition, the upper metal mold 1u is released. After this release, the surface of the primary molded film h1 is cooled slightly by environmental air. Thereafter, as shown in Fig.5, the air suction unit 4 for the lower metal mold 1b is stopped, and the switch valve 3 is switched to the air exhaust unit 5. As the air exhaust unit 5 is operated, the primary molded film h1 floats upward by the air pressure and is dismounted from the lower metal mold lb. Next, the robot R transports the primary molded film h1 to the site of metal molds lu and lb of another thermal treatment molding machine PH. The thermal treatment molding machine PH has the structure and function basically similar to those of the primary molding machine P1, and the structure of the upper and lower metal molds lu and lb is basically similar. The temperature of the upper metal lu mold has been set to 380 °C, and that of the lower metal mold lb has been set to 250 °C. The primary molded film hl is disposed at the intermediate of the upper and lower metal molds to press it for about 30 seconds for thermal fixation. This thermal treatment is performed to crystallize the amorphous primary molded film H1 in order that a predetermined density of a PPTA film is given to an amorphous hard film Ff. At this stage, the primary molded film h1 is eventually transformed into a molded film having an intrinsic mechanical intensity. In the following, this molded film is called a thermal treatment molded film hn. In this hot-press process, when the primary molded film h1 is placed on the lower metal mold 1b, the switch valve 3 for the lower metal mold 1b is switched to the air suction unit 4. At the same time, the air suction unit 4 is operated to tightly contact the primary molded film h1 with the lower metal mold 1b. Thereafter, the upper metal mold 1u is lowered to completely clamp the primary molded film h1 by the upper and lower metal molds 1u and 1b. Under this condition, the thermal treatment described above is performed. In releasing the metal molds, the switch valve 3 for the upper metal mold 1u is switched to the air exhaust unit 5. While the air exhaust unit 5 operates, the cylinder S is raised to release the metal molds. In this case, similar to the primary molded film process, the molded film tightly attached to the upper metal mold 1u is peeled off by exhaust air from the upper metal mold 1u, and since the air suction unit 4 for the lower metal mold 1b continues its operation, the upper metal mold 1u is released while the thermal treatment molded film is sucked to the lower metal mold 1b. After releasing the upper metal mold 1u, the film is rapidly cooled by environmental air down to about 250 °C. Thereafter, the air suction unit 4 for the lower metal mold 1b is stopped, and the switch valve 3 is switched to the air exhaust unit 5 to operate it and dismount the thermal treatment molded film. This molded film is transported by the robot R to the trimming press PT to trim unnecessary areas of the molded film to complete the film in the shape of, for example, a diaphragm. A PPTA gel film Fg used in the second molding method, contains water at least 50 % or more, preferably 80 % or more, as a swelling agent, and has a thickness of about 140 µ. The gel film Fg is placed in an oven at about 180 °C to evaporate water contained in the gel film to thus obtain an amorphous hard film Fg thus thermal treated in an unrolled state and having a density lower than an intrinsic value. This hard film Ff has a modulus in tension of about 70 % and a thickness of 25 µf. As shown in Fig.7, this hard film is guided into a 50 µ gap between heater blocks 6 heated to 250 to 330 °C to preheat the film and further increase the modulus in tension. After this preheating, the film Ff is fed to the site of mold metals 1u and 1b. The upper metal mold 1u has been heated to 380 °C, and the lower metal mold 1b has been heated to 250 °C. The film Ff is hot-press molded and at the same time thermally fixed for about 30 seconds by the upper and lower metal molds. Releasing the upper and lower molds is effected in the same manner as in the first molding method. Like the second molding method, water as the swelling agent contained in a PPTA gel film Fg is evaporated in an oven at about 180 °C to obtain an amorphous hard film Fg having a density lower than an intrinsic physical value. As shown in Fig.8, without performing a preheat process, this film Ff is guided to the site of metal molds 1u and 1b. The upper metal mold 1u has been heated to 380 °C, and the lower metal mold 1b has been heated to 250 °C. The film Ff is hot-press molded and at the same time thermally fixed for about 30 seconds by the upper and lower metal molds. Releasing the upper and lower molds 1u and 1b is effected in the same manner as in the first molding method. In all of the first to third molding methods, the switch valves 3, air suction and exhaust units 4 and 5 connected to the upper and lower metal molds 1u and 1b have been operated. If a molded film can be smoothly dismounted without operating the air suction and exhaust units, which depends upon the molding conditions such as a shape of a molded film, the valves 3 and units 4 and 5 are not required to be operated, or they are not necessary at all. The embodiments of the invention each have a particular characteristic. The first molding method is suitable for a deepest press and a complicated shape of a molded film. The second and third molding methods become, in this order, suitable for a shallower press and a simpler shape of a molded film. From the viewpoint of facilities at the film molding work site, the third molding method requires most simple facilities, and the second and first molding methods in this order require more complicated facilities. When a loudspeaker diaphragm was molded by the above-described molding methods, as compared to one mold cycle time of about 5 minutes of a conventional molding method, one mold cycle time of the first molding method was 1 minute and 30 seconds to 2 minutes and 30 seconds being shorted by 50 to 70 %, and one mold cycle time of the second and third molding methods was 30 seconds being shorted by 90 %. As a result, the productivity was improved about two times to ten times. According to the film molding method of this invention, one mold cycle time can be shortened considerably as compared to a conventional molding method, and therefore the productivity can be improved greatly. There is no deformation of molded products, allowing to obtain products of an excellent quality.

Claims: A film molding method of hot-press molding a film made of substantially para-oriented aromatic polyamide having a modulus in tension of 600 Kg/mm 2 or more, particularly a typical film made of polyparaphenyleneterephtalamide, into a predetermined shape, said method comprising: a first step of evaporating water from a polyparaphenyleneterephtalamide film containing water at least 50 % or more, or preferably 80 % or more, as a swelling agent, and obtaining an amorphous film having a density less than a predetermined density and a modulus in tension of about 70 %; and a second step of hot-press molding said film for a predetermined time period by a male type metal mold and a female type metal mold heated up to at least 330 °C, or preferably 350 °C or more to crystallize said film to have a predetermined density specific to a polyparaphenyleneterephtalamide film, and obtaining a molded product of said film. The film molding method according to claim 1, wherein said evaporating is performed by placing said water containing film in an oven. The film molding method according to claim 2, wherein the oven temperature is about 180 °C. The film molding method according to any preceding claim, wherein said film obtained by said first process is preheated at 250 °C or more to 330 °C or less after said first process. A film molding method of hot-press molding a film made of substantially para-oriented aromatic polyamide having a modulus in tension of 600 Kg/mm 2 or more, particularly a typical film made of polyparaphenyleneterephtalamide, into a predetermined shape, said method comprising: a first step of hot-press molding a polyparaphenyleneterephtalamide film containing water at least 50 % or more, or preferably 80 % or more, as a swelling agent, by using a first pair of male-female metal molds heated to 220 °C or less, or preferably 190 °C or less for a predetermined time period, and producing a primary molded product of said film; and a second step of hot-press molding said film for a predetermined time period by using a second pair of male-female metal molds heated up to at least 330 °C, or preferably 350 °C or more to crystallize said film to have a predetermined density specific to a polyparaphenyleneterephtalamide film, and obtaining a molded product of said film, wherein the male and female molds used in the first step are not identical with the male and female molds used in the second step. The film molding method according to any preceding claim, wherein said male type metal mold and said female type metal mold are released by attaching said polyparaphenyleneterephtalamide film molded product to one of said male type metal mold and said female type metal mold in a sucked manner.

Inventors: SAKAMOTO YOSHIO, SAKAMOTO, YOSHIO
Applicants: KENWOOD CORP, KABUSHIKI KAISHA KENWOOD
Application year: 1994
Main classification: B29C51
IPC codes: B29C51, B29C35, B29K77, H04R7, H04R31, B29L31